Shot peening is a widely used cold surface treatment method for materials. Its basic principle involves using high-speed shot particles—such as ceramic beads, steel balls, or glass beads—which strike the workpiece surface with the force of countless small hammers, inducing elastic–plastic deformation in the material. This process has the following significant effects on the material surface:
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1. Incorporation of residual stress and work hardening
Under continuous impact from projectiles, the material surface undergoes plastic deformation and develops a large number of microscopic pits. As the undeformed grains beneath the surface attempt to return to their original configuration, a uniform residual compressive stress layer forms on the material surface. Simultaneously, the surface metal experiences cold work hardening, significantly enhancing the strength and hardness of the material surface.
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2. Modifying the surface microstructural organization
The surface layer subjected to cyclic plastic deformation through shot peening undergoes profound changes in its microstructural organization. Research indicates that shot peening significantly refines the subgrains, substantially increases the dislocation density, and enhances lattice distortion. Furthermore, for certain heat-treated alloys (e.g., carburized and quenched gears), shot peening can also induce the transformation of residual austenite in the surface layer into martensite, thereby generating additional compressive stress due to the volumetric expansion associated with this phase transformation.
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3. Enhance fatigue resistance and corrosion resistance
The presence of residual surface compressive stress can effectively counteract the tensile stresses induced by external alternating loads, thereby inhibiting the initiation and propagation of fatigue crack sources. This significantly enhances the material's bending fatigue strength, contact fatigue strength, and overall fatigue life. Additionally, the densified surface layer and compressive stress layer improve the material's resistance to stress corrosion and high-temperature oxidation.
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4. Modifying surface morphology and roughness
Shot peening causes the originally smooth metal surface to be textured, creating tiny metal peaks and pits, thereby increasing the surface roughness. Generally, as the shot peening intensity increases, the surface hardness decreases, and the shot size increases, the surface roughness value also rises. However, excessive shot peening may induce significant stress concentrations in the deeper pits, which can actually undermine the strengthening effect.
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5. Impact on size and shape
Due to plastic deformation occurring on the surface, which creates a compressive stress layer, the metal on the sprayed surface is extruded, potentially causing a slight increase in the macroscopic dimensions of the workpiece.
In conclusion, shot peening demonstrates significant effectiveness in improving the mechanical properties of materials; however, in practical applications, it is essential to properly control process parameters such as shot peening intensity, shot size, and coverage rate to achieve the desired strengthening effect while avoiding the adverse consequences associated with excessive surface roughness.